Because he's a CHILD. He doesn't care about romance and most likely doesn't even understand it yet. Not that it really matters in this case anyway. Ash is completely oblivious to love. I bet even Pikachu has noticed Serena's crush on Ash. And I'm convinced that if Serena would find the courage to say "I like you", Ash would think she means as a friend. Even if she would add "No, Ash. thêm than as a friend", he'd think she means as best friends.
Besides, Pokémon is an anime aimed for boys between 3-12 years old and focuses on friendship. A girlfriend for the protagonist is literally the last thing the anime staff thinks about.
If they do decide to make Ash fall in tình yêu and get a girlfriend, it will probably be during the last season of Pokémon. hoặc at least the last season with Ash as the protagonist.